1. Field of the Invention
The present invention relates to an FIR filter apparatus (Finite Impulse Response Filter apparatus) for calculating impulse response coefficients and for performing a filtering process using the calculated impulse response coefficients, and further relates to an audio apparatus provided with such FIR filter apparatus and an FIR filter program.
2. Description of the Related Art
Various sorts of apparatuses are known for adding resonance tones to audio signals generated by an electronic musical instrument and audio apparatus, wherein the resonance tones include reverberation tones produced at a place including a hall, where musical tones are generated.
In general, a resonance tone adding apparatus receives digital audio signal data and performs a filtering process of a digital filter on the received digital audio signal data, thereby generating resonance tones. To perform the filtering process, FIR filter and/or IIR filter (Infinite Impulse Response FILTER) are used.
In the case where FIR filter is used, a convolution operation is performed on input audio signal data X(n−k) and impulse responses hn(k) obtained from reverberation characteristics of a hall, whereby a resonance toney(n)=X(n−k)×hn(k)is obtained. Increasing the number of impulse response coefficients used in FIR filter, that is, increasing the number of filter taps will make filter characteristics more ideal, but will invite a disadvantage that increases the number of calculations to process the audio signal data.
For example, U.S. Pat. No. 6,782,105 B1 discloses a technique that generates a large number of resonance tones using less number of parameters. More specifically, two FIR filters are connected in series, and parameters (filter coefficients) extending for a long time duration are intermittently used in the former stage of FIR filter. Meanwhile, the parameters (filter coefficients) extending for a short time duration are used more frequently in the latter stage of FIR filter.
US Patent No. 2008-0247553 A1 discloses a technique, in which two FIR filters having different filter characteristics are used, and output of the former stage of FIR filter is supplied to the latter stage of FIR filter.
As described above, a series connection of FIR filters can generates a number of resonance tones and reflection tones. However, the above techniques have disadvantages that filter coefficients must be prepared for two filters, and a circuit scale will be made large and data volume to be processed will increase.